X-box Binding Protein 1 Regulates Endothelial Inflammation and Blood-retinal Barrier Homeostasis in Diabetic Retinopathy

Abstract

Purpose: : Endoplasmic reticulum (ER) stress has been widely implicated in chronic inflammatory diseases, such as diabetes. X-box binding protein 1 (XBP1) is a central coordinator of cellular responses during ER stress. Previously we reported that preconditioning with ER stress mitigated retinal endothelial inflammation via activation of XBP1. Further, we found that retinal expression of XBP1 was markedly reduced in diabetic mice. The objective of this study is to investigat the role of XBP1 in retinal inflammation and vascular leakage associated with diabetic retinopathy (DR).

Methods: : Endothelium-specific XBP1 deficient (XBP1EC-/-) mice were generated by using the Cre-loxp system. Diabetes was induced by intraperitoneal injection of streptozotocin (STZ). Expression of adherent molecules and tight junction proteins, leukostasis and vascular permeability were evaluated in diabetic XBP1EC-/- and wild type (XBP1EC+/+) mice. Mouse brain endothelial cells were isolated from both XBP1EC-/- and XBP1EC+/+ mice for in vitro study. Expression of tight junction proteins and adherent molecules was determined by Western-blot analysis or immunocytochemistry.

Results: : Retinal expression of intracellular adherent molecule-1 (ICAM-1) and vascular adherent molecule-1 (VCAM-1) was significantly increased, in parallel with enhanced leukocytes adhesion to retinal vasculature in diabetic XBP1EC+/+ mice. When compared with XBP1EC+/+ mice, diabetic XBP1EC-/- mice showed much higher levels of adhesion molecules and greater increase in leukostasis in the retina. In addition, retinal expression of zonula occludens-1 (ZO-1), a major tight junction protein, was much lower in XBP1EC-/- mice, indicating impaired blood-retinal barrier. Furthermore, XBP1EC-/- mice showed more severe retinal vascular leakage compared with XBP1EC+/+ mice. In addition, endothelial cells isolated from XBP1EC-/- mice expressed much higher level of ICAM-1, but lower level of ZO-1 after incubation with pro-inflammatory cytokine TNF-α. This suggests that loss of XBP1 sensitizes endothelial cells to inflammation and tight junction damage.

Conclusions: : Our results indicate that XBP1 plays a vital role in regulating endothelial barrier function. Impaired XBP1 signaling may in part contribute to retinal inflammation and vascular leakage in diabetic retinopathy.

Keywords: diabetic retinopathy • inflammation • cell adhesions/cell junctions